They said “both parties will encourage support” for former Belfast Mayor Gavin Robinson in the east of the city where the DUP were left bruised after party leader Peter Robinson lost the seat to Alliance’s Naomi Long.

Former UUP leader Tom Elliot will get the same treatment in Fermanagh & South Tyrone where Sinn Fein’s Michelle Gildernew won in a poll so tight the result ended up in court.

In North Belfast both parties will encourage support for Nigel Dodds to fight off any challenge from Sinn Fein’s Gerry Kelly while his party colleague Conor Murphy will face Ulster Unionist Danny Kennedy in Newry & Armagh.

Peter Robinson described the pact agreement as “the most comprehensive electoral agreement between our two parties in the last 29 years”.

But reacting to the news, Sinn Féin MP Conor Murphy said “nationalists, republicans and other progressives should come together to ensure maximum representation for parties committed to defending the core public services of health, education and the welfare state; parties which are unequivocally for equality and which are totally opposed to sectarianism, racism and homophobia”.

He added: “Martin McGuinness will be speaking to the SDLP leader Alasdair McDonnell at the earliest opportunity to explore an appropriate and strategic response to this pro-Tory alliance.

“We can only achieve this if the SDLP is prepared to put progressive, forward looking politics before their own narrow self-interests.”

For months most experts have been predicting that not only will neither the Conservatives nor Labour be able to command an overall majority in the Commons or form a coalition with one of the smaller parties - it appears that one or other of them will have to assemble some form of patchwork alliance involving multiple parties, or run as a minority administration with no guarantee it can get its business through the House.

Mr Robinson added: “Those who support the Union will recognise the significance of this pact.”

He said the agreement is the product of discussions lasting six months.

The party leader added: “I want to see unionists co-operating. Too often divisions are manufactured to create difference. Grassroots unionists want to see us working together to maximise the unionist vote.

“Such an approach is not just desirable, but it has been proven to be effective in increasing turnout amongst unionists.

“With a predicted hung parliament, I am calling on all unionists to unite behind these agreed candidates and maximise the pro-union voice in the House of Commons.”

In a reference to Sinn Fein not taking their seats, Mr Robinson claimed: “They leave their constituents without a voice in Parliament. Others do not represent unionism.

“All shades of unionism can lend their vote to these agreed candidates in the knowledge that their action could increase the number of unionists in the House of Commons and reduce the number of non-unionists returned.”

He said it had not been possible to include the constituencies of Upper Bann and South Belfast in the pact. Tonight a well placed source told Belfast Live pacts on those two constituencies were not agreed as the DUP believed two agreed candidates, but not from either party was the best way forward, however the UUP would not agree to the move.

Nationalist SDLP leader Dr Alasdair McDonnell is the incumbent in South Belfast while the DUP’s David Simpson holds Upper Bann. Sinn Fein is running their high profile former city mayor Mairtin O Muilleoir against Dr McDonnell.

UUP leader Mike Nesbitt said: “We began these discussions in October proposing an anti-abstentionist arrangement for Fermanagh & South Tyrone and North Belfast (against Sinn Fein).

“Since then it has become clear that there is potentially a once in a lifetime opportunity to take back Newry & Armagh, albeit this will be extremely difficult to achieve. Our support for the DUP in East Belfast should ensure an additional pro-Union MP for the city of Belfast in the next mandate.”

Reacting to the news, East Belfast MP Naomi Long said the “DUP clearly now know that Gavin Robinson cannot win against Alliance in a fair fight, so they have resorted to this anti-democratic move in a desperate attempt to strengthen their position”.

She added: “Many UUP voters were already disillusioned by the party’s involvement in whipping up sectarian tensions around flags and parades and this decision will only serve to further alienate them. This decision has sounded the death knell for the UUP in East Belfast, who have abandoned their voters without gaining any truly winnable seats in return. It has also confirmed what has long been the case – if you vote UUP in East Belfast, you get the DUP.”